Patients with low back pain will undergo ultrasound imaging of the lateral abdominal wall to measure resting and contracted thickness of the transversus abdominis muscle. Following the pre-intervention measurement, patients will receive one of two physical therapy interventions (side lying thrust joint manipulation or side lying non-thrust oscillations into slight rotation) which will have been randomly assigned. Immediately after intervention, ultrasound imaging will be repeated on the resting and contracted thickness of the transversus abdominis.